Straightforward smartphone assay for quantifying tannic acid in beverages based on colour change of Eu3+/polyethyleneimine complex

•Straightforward assay for sensing tannic acid using a smartphone.•Polyethyleneimine-derivatized polymer was synthesized via sonication at room temperature.•Fluorescence of the polyethyleneimine-derivatized polymer was quenched effectively by tannic acid with the aid of Eu3+ ions.